Power take off devices are commonly used with power sources such as vehicle engines or motors or vehicle transmissions to provide power to a driven accessory. For example, power take off devices are commonly used on farm tractors to provide power to accessories such as mowers or other accessories, on highway vehicles to provide power to accessories such as winches or pumps or blowers, on industrial equipment to provide power to pumps and blowers and other accessories, and on marine vehicles to provide power to pumps and propulsion devices and other accessories.
A typical power take off device can include a housing, an input gear that extends out of the housing, and an output shaft that also extends out of the housing. When the power take off device is mounted to a source of rotational power, the input gear meshes with and is driven by a drive gear that is driven by the power source. The input gear is directly or indirectly drivingly connected to the output shaft within the housing to transmit power from the input shaft to the output shaft. An accessory is drivingly connected to the output shaft and in turn is powered by the power source. The power take off device may provide a constant speed ratio or a speed reduction or a speed increase between the input gear and the output shaft and may include a clutch to engage and disengage the drive connection between the input gear and the output shaft.
In certain uses of power take off devices, it would be advantageous to provide two or more speed ratios rather than a single constant speed ratio between the input gear and the output shaft. For example, when a power take off device is used to propel a blower, it would be advantageous for the power take off device to provide a gear reduction that provides relatively lower speed and relatively greater torque of the output shaft to overcome the relatively high inertia forces of the blower at start up. Once the blower has reached a particular rotational speed, it would be advantageous for the power take off device to increase the speed of the output shaft and of the blower. This two ratio or two stage or two speed approach could reduce the stress on a clutch within the power take off device during such start up of the blower. Another application in which it would be advantageous for the power take off device to rotate the output shaft first at a slower speed and then at a faster speed is when the power take off device is used to power a winch. In this application, it would be advantageous to rotate the winch at a greater speed when the winch cable is being played out to reach a load and to then rotate the winch at a slower speed with increased torque to retrieve the load.
It would also be advantageous in certain uses of power take off devices to provide a large speed reduction gear ratio between the input gear and the output shaft while achieving a small compact size. For example, in many uses of power take off units on vehicle transmissions, the space available for the power take off unit is limited, yet it can be desirable to provide a large speed reduction gear ratio that provides a correspondingly large torque increase. In conventional power take off units it can be difficult to achieve a large speed reduction gear ratio while achieving a compact size. This is because a large speed reduction gear ratio can require at least one relatively large diameter gear that has a longitudinal axis laterally offset relative to the axis of a small gear that drives the large gear.
As used herein, a power take off device means a mechanical or mechanical—fluid device that transmits power directly or indirectly from a power source to a driven accessory. A removable power take off device is a power take off device that has releasable fasteners that removably connect the device to the power source. A transmission mounted power take off device is a power take off device that is mounted to a transmission that transmits power from a primary power source such as a motor or engine of a vehicle to a primary powered device such as the propulsion component of the vehicle, in a manner such that the power take off device transmits power from the primary power source through the transmission to the driven accessory independently of transmission of power to the primary powered device.